1. Field of the Invention
This invention relates to cermet conductor films and more particularly to compositions useful for producing conductive patterns adherent to substrates.
2. Description of the Prior Art
Increased use of glass sheet as a substrate for printed circuitry and gas discharge displays has impelled commercial development of compatible conductor metallizations. Traditional metallizations employing noble metals such as silver, gold, platinum, or palladium have been found to be poorly suited for use on glass substrates. For example, the firing temperature of these compositions is generally too high to be withstood by substrates constructed of soda lime glass. In addition, noble metal compositions are prone to sputtering and glow discharge and are among the costliest metals in common use. Other materials utilizing a base metal conducting phase such as nickel, as disclosed by Patterson in U.S. Pat. No. 3,943,168, overcome some of these deficiencies, but still suffer from certain drawbacks. They are, for example, still relatively expensive and they tend to suffer from poor moisture resistance (the latter being the result of a high B.sub.2 O.sub.3 content).
Another shortcoming of nickel based compositions is evident where they are used to make gas discharge display devices. As is generally known, the operating characteristics of gas discharge displays are significantly improved when cathode surfaces are formed of high density nickel. Commercially available metallizing compositions contain frit glasses which have been incorporated to provide acceptable cohesive properties and to provide good adhesion to a substrate. However, this glass content at the same time makes it impossible for a film to possess the aforesaid desirable dense nickel surface. It is, therefore, a usual practice to plate cathode surfaces of gas discharge displays with nickel metal in order to obtain optimum operation. Unfortunately, the plating process frequently degrades the film's adhesion to the substrate.
Aluminum is another base metal which is much more inexpensive than nickel. (Aluminum costs approximately 4.7.cent. per cubic inch versus nickel which costs approximately 65.cent. per cubic inch.) However, aluminum yields relatively poor thick film patterns having relatively high resistivities because of, among other things, the extensive formation during firing of aluminum oxide. If one adds glass to finely divided aluminum, as taught in Dates et al.'s U.S. Pat. No. 3,484,284, one can lower the resistivity somewhat but one still obtains a thick film pattern having a relatively high resistivity. In addition, the aluminum-glass conductor composition possesses a resistivity which is very sensitive to firing temperatures, i.e., has a very narrow firing range (firing window) wherein one can obtain conductor compositions possessing a desirable resistivity.
Because of the low cost of aluminum it would be very advantageous if one could further lower the resistivity of aluminum based conductor compositions as well as broaden the firing window of said compositions. One avenue of research that those skilled in the art would immediately reject is the addition to aluminum based compositions of alloying metals having a higher resistivity than the resistivity of aluminum. As those skilled in the art know, the alloying of two metals usually results in an alloy which possesses a resistivity at least greater than the resistivity of the more conductive component and often greater than the resistivity of the less conductive component as well.